While many communication devices such as conventional landline and cellular telephones include a keyboard for the input of a numeric telephone number, size constraints typically limit the number and arrangement of keys. Typically telephones include a small numeric or “touch tone” keypad or dial pad configured according to international standard ITU E.161 established by the International Telecommunications Union (ITU) and/or according to ISO 9995-8 established by the International Organization for Standardization. The keypad layout includes 12 keys representing numerals 0-9 and the symbols “*” (called the “star key” or “asterisk key”) and the. “#” (called the “octothorpe”, “number sign”, “pound key”, or “hash key”.) The standards provide for the assignment of the 26 letters A-Z to the number keys of a numeric keypad in support of, among other things, the entry of alphabetic or “alpha” characters.
Because each key may represent a number in addition to three or four letters of, for example, the English alphabet, several technique have been used to determine the symbol (number or letter) that is being input or selected by a user. One method known as “multi-tap” requires that a user depress a key a number of times corresponding to the order (or “ordinality”) of the letter being selected. In one such scheme, the keypad may be operated either in a numeric input mode in which each keystroke is interpreted as the corresponding number or in an alpha mode in which the number of times a key is depressed is used to determine the letter being input. Another method known as predictive text may use a dictionary to match a sequence of numeric inputs to corresponding numeric codes of dictionary entries. For example, the name “BOB” would correspond to a numeric sequence of 2-6-2.
The predictive method works relatively well for short text messages using a limited dictionary of words wherein ambiguity between dictionary entries based on their numeric equivalence is minimal. However, the predictive method is not as efficient when the dictionary includes many similar words and/or combinations of words and numbers. In such case, disambiguation between and among the various dictionary entries (e.g., listings in a directory) that are represented by the same sequence of keystrokes may become burdensome and limit system utility in providing a convenient user interface.